Method of electroplating aluminum



Feb. 14, 1961 D. K. HANINK ETAL 2,971,899

METHOD oF ELECTROPLATING ALUMINUM Filed sept. 1o, 1957 United States Patent O METHOD F ELECTROPLATING ALUMINUM Dean K. Hannk, Indianapolis, Ind., and Albert A.

Shoudy, Jr., Royal Oak, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Sept. 10, 1957, Ser. No. 683,066

3 Claims. (Cl. 204-29) This invention relates to a process for electrodepositing aluminum and aluminum alloys by means of a molten salt electrolyte.

It is desirable in many instances to apply a coating of aluminum to various articles, particularly those which are to be subjected to elevated temperatures and corrosive conditions, in order to improve the physical properties of these articles. The term aluminum, as used herein, includes aluminum base alloys containing at least 80% aluminum. When such a coating is applied and subsequently treated so as to form form a thin diffusion layer of an alloy of aluminum with the base material, the resultant product is not only highly oxidation-resistant at elevated temperatures, but the protective alloy `layer is sufciently adherent and ductile so that it will not spall or ake when subjected to thermal shock, cyclic heating and the like.

A process for producing such corrosion-resistant alloy layers is described-and claimed in co-pending patent application Serial No. 615,417, which was tiled on October ll, 1956, in the names of Dean K. Hanink and Erwin R. Price, and is owned by the assignee of the present invention. Briey, this process involves applying a coating of aluminum or aluminum alloy to the base metal, such as by a hot dip operation, and thereafter diffusing a portion of the aluminum into the base metal and forming an alloy therewith. Oxide penetration of the base metal is thereby eliminated. In order to provide a satisfactory thin, relatively ductile layer of the oxidationresistant alloy, however, it is necessary to remove a portion of the as-dipped aluminum coating, such -as by an acid etch, and subsequently diffuse the remaining aluminum into the base metal by a relatively extended thermal treatment. The thin protective alloy layer thus produced is particularly benecial in applications where ductility and thermal shock resistance are desirable.

While the foregoing process has proved to be highly satisfactory and is employed commercially today, it nevertheless has the disadvantage of requiring an acid treatment and a relatively long period of treatment at an elevated temperature to dilfuse the aluminum or aluminum alloy coating layer into the base metal. Such a process is therefore necessarily relatively time-consuming and expensive.

Accordingly, a principal object of the present invention is to provide a process and apparatus for forming a thin, ductile, oxidation-resistant layer of au alloy of aluminum with a base metal by means of an electroplating process which eliminates the aforementioned disadvantages.

A further object of this invention is to provide a metal surface with a protective aluminum alloy layer which possesses suicient ductility to yield with the base metal when the latter is expanding or contracting. Another object of this invention is to provide an integral alloy layer of aluminum with a base metal on the surface of the base metal which aiords effective protection against high-temperature oxidation.

2,Q7l,899 Patented' Fea. 14, 19m

ICC

These and other objects are attained in accordance with the present invention by a process whereby a thin, integral, ductile alloy layer of aluminum and a base metal is formed on the surface of the base metal by electrodepositing aluminum onto the base metal rom a molten salt electrolyte. The fused or molten salt electrolyte preferably comprises sodium chloride, potassium chloride, cryolite and aluminum fluoride.

Other objects, features and advantages of this invention will become more apparent from the following description of a preferred embodiment thereof and from the accompanying drawing, which contains a schematic sectional view of an apparatus which can be used to electrodeposit aluminum in accordance with the invention.

Ferrous and nonferrous metal articles of any shape can be coated with aluminum by the method of this invention. The process can be a continuous one if desired, especially if the metal being treated is in the form of sheet, Awire or tube stock.

When an article formed of steel or Vother ferrous metal, for example, is to be coated, it is preferably first degreased in any suitable manner, such as by means of an alkali cleaner or a suitable organic solvent. After degreasing the metal, if it is severely rusted or scaled, it

' may be advantageously pickled in a hydrochloric acid solution in known manner. After the cleaning operation, the metal article may be immersed in a flux such as one composed of 32 parts of zinc chloride, 8 parts of ammonium chloride and 60 parts of water, all measurements by weight. The foregoing flux is given as a typical example of a zinc chloride type iiux which may be used.

After treating the metal article in the uxing medium just described, in accordance with a preferred embodiment of our invention, the article is immersed in a molten salt bath. The salt is maintained at a temperature somewhat above the melting point of aluminum. The article is preferably allowedto remain in contact with the fused salt for a suicient time to heat the metal to or abovev the melting point of aluminum. When the article has been suiiciently heated, it is subjected to a negative potential and the molten salt bath is electrolyzed for a short time, thereby depositing aluminum onto the surface of the heated article.

The fused salt, in addition to serving as an electrolyte, must also function as a ux which cleanses the surface of the metal and permits the aluminum to wet and alloy with the metal on which it is deposited. The following is a specific salt bath that we have found to be highly satisfactory, the proportions being by weight:

The exact composition of the salt bath is not particularly critical and the proportions of aluminum fluoride, cryolite, sodium chloride and potassium chloride may be varied somewhat from the above. For example, the aluminum iinoride can vary from about 0.5 to 12%, the cryolite from 8.0 to 20%, the sodium chloride from 25 to 45% and the potassium chloride from 37 to 57%. The bath composition usually preferred is one that will become molten when heated to 1200* F. or somewhat lower. While in the foregoing examples the double salt NasAlF (cryolite) is given, it should be understood that an equivalent amount of this component may be supplied in the form of the single salts, sodium uoride and aluminum fluoride. We have found, however, that it is essential to provide an excess of aluminum uoride over that of the cryolite ratio in order to obtain the desired results. l

During operation the temperature of the fused salt bath is maintained at approximately 1250" F. to 1600o F. Below 12.50 F. the molten salt electrolyte becomes less active with respect to fluxing ability, while above 1600" F. it becomes highly volatile and excessivelosses due to vaporization may occur. In general, a temperature of 1300 F. to 1400 F. is preferred.

. Electrolysis of the molten or fused salt bath with 4the metal under a negative potential deposits aluminum onto the surface of the base metal article. As the aluminum is deposited it also is diffused into the surface of the heated metal. Preferably the rate of deposition of the aluminum is so regulated that the amount diffusing into thesurface of the metal is generally'equal to that being deposited, In this manner, substantially no separate overlay of aluminum is formed. After electrolyzing the molten bath for a suicient time to deposit the desired quantity of aluminum, the passage of current through the salt bath is terminated. In some instances, to insure complete dilusion of the aluminum coating` into the surface of the base metal, it is desirable to leave the coated metal article in contact with the salt bath for a short interval of time after the termination of electrolysis. This diffusion can also be conveniently accomplished using a separate furnace which is at a temperature generally similar to the salt bath. Thereafter, the coated metal article is cooled to a temperature of approximately 600 F., preferably in air, and subsequently washed with water in order to remove salt adhering `to the surface thereof. Water or other quenching media also may be employed to cool the coated article. The excess salt electrolyte likewise may be removed by passing the coated article through rollers, this procedure further providing the coated product with a desirable finish.

Y The steps of degreasing, pickling and immersing in the zinc-chloride type of flux are not essential to the process, unlike the step of heating prior to the electrolysis opera` tion. Heating of the article in the described fused salt bath'will normally provide a clean surface on the article without further treatment. Thefmetal article may be preheated, if desired, before immersing in the fused salt bath. However, in most instances, it is preferred to heat the article to be coated under such conditions that the surface thereof is not oxidized. Accordingly, preheating in a non-oxidizing or reducing atmosphere furnace Vsuch as one employing hydrogen, .Drycolene, etc., can be employed. The preheating temperature is preferably generally similar to the temperature of the fused salt bath.

If the article to be coated is preheated to the temperature of the fused salt bath in a reducing atmosphere and is free of oxides andother foreign matter, the time of immersion in the salt bath prior to electrolysis may be as little as one or two seconds. For parts which have complicated shapes and complex recesses, a longer time may be required to insure that the fused salt has thoroughly contacted all the surfaces of the metal. When the metal article has extensive oxide formation on the surface thereof, longer times will be required in order toy cleanse the surface and prepare it for the deposition of the aluminum. lf va separate preheating step is not employed prior to electrolysis, the article to be coated should be immersed in the molten ux electrolyte a sulficient interval of time to bring the metal to approximately the bath temperature. Of course, the duration of such heating will depend upon the dimensions of the metal lzrtilcle and on the size and thermal efficiency of the salt at Y The time of electrolysis of the metal article inthe fused salt bath may vary from as little as a few seconds up to a` few minutes, depending upon the complexity of the recesses and the dimensions of the article, the current density used, etc. A relatively short period of electrodeposition Yof the aluminum on the base metal surface is desirable in order to prevent excessive formationinthe protective layer of a high-aluminum outer phase in which the aluminum has not been'suiciently diffused into the surface of the base metal.

Although in most instances the above-described diffusion in the fused salt bat-h is sufficient to provide a satisfactory oxidation-resistant protective surface layer of an alloy of aluminum with the base metal, for particular applications it may be desirableV t further diffuse' the coating into a base metal, especially when it is desired to electrodeposit the aluminum at a rate higher than it is convenient to diffuse the aluminum into the base metal article While in the salt flux. This additional diffusion may be induced into the surface of an article formed from a nickel base alloy, for example, by heating the coated article at a temperature of, approximately l700` F. to 2350 F. for one to six hours. Optimum resultsappear to be produced with a two-hour diffusion heat treatment at 2 10 0 F., although a heat treatment for live hourslat 1800 F. is also satisfactory for further diffusing, the formed aluminum-nickel alloy layer.

The herein-described process isparticularly useful in providing a corrosion-resistant protective surface layer on high-temperature alloy components, such as turbine buckets and nozzle guide vanes, of gas turbine engines since these articlesl are subjected to extended periods of service at elevated temperatures under variable stress conditions. When such components are formed of certain` high-temperature nickel base alloys and cobalt base alloys, they possess excellent strength under most high-tempera ture surface conditions. However, the durability of these turbine blades may be materially improved because ofl thel increased resistance to oxide penetration afforded by a protective surface ,layer` produced in the above-de scribed manner. The diffused aluminum-nickel alloy or aluminum-cobalt alloy layer thus formed is tough, resilient and possesses good ductility. Y Various high-temperature nickel base alloys and cobalt base alloys, such as those described in United States patent application Serial No.. 615,417, referred to above, areappropriate for use in this process. An alloy of the type disclosed yin United States Patent No. 2,688,536 Webbere et al. is especially benefited by the foregoing treatment, Such an alloy comprises approximately 0.06% to 0.25% carbon, 13% tov 17% chromium, 4% to 6% molybdenum, 8% to 12% iron, 1.5% to 3% titanium, 1% to 4% aluminum, 0.01% to 0.5% boron and the balance substantially all nickel. For some applications, the aluminum content may be increased to approximately 6% and the iron content may be as low as 0.1% or as high as 35%. The alloy usually should not contain morethan 20% iron, however. Normally, manganese and silicon not in excess of 1% each are also included in the alloy. Y

An aluminum-nickel or aluminum-cobalt alloy protective layer,for example, should be extremely thin. In general, this layer should have a thickness of approximately 0.0005 inch to 0.0025 inch. A diffusion layer of about 0.0012 inch to 0.0020 inch in thickness is preferred, however, with a layer thickness of about approximately 0.0015 inch being considered optimum at the present time.

An apparatus which may be advantageously employed to practice this invention is shown in the accompanying drawing. This apparatus comprises a rectangular open furnace 10 having an outer steel shell or casing 11 and an inner cavity 12. The inner surface of the casing 1l is lined with refractory insulating material 14, such as ceramicor the like. A covering layer of a non-corrosive material 16, such as silicon carbide, is employed to pro tect the insulation from corrosion during operation of the furnace. The furnace contains a pool or bath 18 of Va fused or molten salt containing an aluminum salt below which lies a layer 20 ofvmolten aluminum. An elongated copper bar 22 is disposed above the furnace 10 and insulated therefrom at 24. workpiece 26 t be plated is suspended therefrom, being `-suitably 'connected 'thereto so as to provide an electrical communication between the bar and the Work to be coated. A source of direct electric current 28 is connected by suitable leads 30 and 32 to the outer steel shell 11 of the furnace and to the copper bar 22, respectively, so as to impose a negative potential on the copper bar 22. This bar positioned over the salt bath 18 serves as a contact for the metal articles being plated and is therefore the cathode in the electrical circuit.

The molten layer of aluminum 20 below the molten salt 18 in the furnace permeates the silicon carbide bricks 16 and the insulation 14 to contact the outer steel shell 11. This aluminum penetration, which is schematically indicated at 21, provides a continuous contact of the layer of aluminum 20 below the salt bath 18 and the steel shell 11 such that the aluminum layer below the molten salt electrolyte functions as an aluminum anode in the bath. The silicon carbide lining 16 should have sucient openings therein below the ux layer and the refractory insulation 14 should have suicient porosity to permit the molten aluminum to penetrate the lining 16, permeate the underlying insulation 14, and contact the steel shell 11. Additional electrical communication between the aluminum layer 20 and the steel shell 11 can be effected by the carbon electrode 34 in the lower wall of the furnace. It is advantageous to employ an aluminum anode in the electroplating bath since the aluminum ion concentration thereby remains relatively constant due to anodic solution of the aluminum. Should a separate steel anode be employed, periodic additions of aluminumluoride, cryolite or both would have to be made to the molten bath in order to maintain the proper aluminum ion concentration.

The current density used during electrolysis is not particularly critical and can range from 0.1 ampere to 30 amperes per square inch, for example. A complex part will have many high current density areas which induces uneven plating and consequently uneven diffusion. Thus, a relatively low average current density should be employed for such parts. In addition, excessive potassium and sodium plate out on the article, interfering with the deposition of the aluminum when an excessive current density is employed. The rate the aluminum diffuses into the base metal at the specific temperature employed and the complexity of the part will determine the optimum rate of electrodeposition.

Although the present invention has been illustrated in connection with certain specific examples thereof, it is not intended to be limited thereby except as defined by the following claims.

We claim:

1. A method of forming an aluminum alloy coating on a surface of an article formed from a metal selected from the group consisting of nickel base alloys and cobalt base alloys, said method comprising heating said article to a temperature of approximately 1250 F. to 1600 F., applying a fused salt bath having a temperature of about 1250 F. to 1600 F. to the surface of said article, said fused salt bath being in contact with a molten aluminum anode and comprising, by weight, approximately 0.5% to 12% aluminum fluoride, 8.0% to 20% cryolite, 25% to 45% potassium chloride and 37% to 57% sodium chloride, imposing a negative potential Aon said article while it is in contact with said salt bath so as to induce a current density thereon of about 0.1 ampere to 30 amperes per square inch to thereby electrodeposit aluminum onto the surface of said article at a rate not substantially greater than the rate at which aluminum diffuses therein, subsequently cooling said article and removing any residual salts from said surface.

2. A method of forming an aluminum alloy coating on a surface of a metal selected from the group consisting of nickel base alloys and cobalt base alloys, said method comprising immersing said metal in a fused salt bath having a temperature of about 1250 F. to 1600 F., heating said metal to said temperature range, said fused salt bath comprising, by weight, yapproximately 0.5% to 12% aluminum fluoride, 8.0% to 20% cryolite, 25% to 45% potassium chloride and 37% to 57% sodium chloride, imposing a negative potential on said metal while it is immersed in said salt bath so as to induce a current density thereon of about 0.1 ampere to 30 amperes per square inch to thereby electrodeposit aluminum onto the surface of said metal at a rate not substantially greater than the rate at which aluminum diffuses into said surface, thereafter cooling said metal and removing residual salts from said surface.

3. A method of forming an aluminum alloy coating on a surface of a metal selected from the group consisting of nickel base alloys and cobalt base alloys, said method comprising heating said metal to a temperature of approximately 1250 F. to 1600 F., placing said metal into contact with a fused salt bath heated to a temperature of about 1250 ,F. to 1600 F., said fused salt bath being in contact with aluminum and comprising, `by weight approximately 0.5 to 12% aluminum uoride, 8.0% to 20% cryolite, 25% to 45% potassium chloride and 37% to 57% sodium chloride, imposing a negative potential on said metal while it is in contact with said salt bath so as to induce a current density of about 0.1 ampere to 30 amperes per square inch on said metal to thereby electrodeposit aluminum onto the surface thereof at a rate not substantially greater than the rate at which aluminum diffuses therein, thereafter maintaining said aluminum coated metal at a temperature of approximately 1250 F. to 1600 F. to additionally effect diffusion of said aluminum into said metal, and subsequently cooling said metal and removing residual salts from the surface thereof.

References Cited in the lle of this patent UNITED STATES PATENTS Hansgirg May 24, 

1. A METHOD OF FORMING AN ALUMINUM ALLOY COATING ON A SURFACE OF AN ARTICLE FORMED FROM A METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL BASE ALLOYS AND COBALT BASE ALLOYS, SAID METHOD COMPRISING HEATING SAID ARTICLE TO A TEMPERATURE OF APPROXIMATELY 1250*F. TO 1600*F., APPLYING A FUSED SALT BATH HAVING A TEMPERATURE OF ABOUT 1250*F. TO 1600*F. TO THE SURFACE OF SAID ARTICLE, SAID FUSED SALT BATH BEING IN CONTACT WITH A MOLTEN ALUMINUM ANODE AND COMPRISING, BY WEIGHT, APPROXIMATELY 0.5% TO 12% ALUMINUM FLUORIDE, 8.0% TO 20% CRYOLITE, 25% TO 45% POTASSIUM CHLORIDE AND 37% TO 57% SODIUM CHLORIDE, IMPOSING A NEGATIVE POTENTIAL ON SAID ARTICLE WHILE IT IS IN CONTACT WITH SAID SALT BATH SO AS TO INDUCE A CURRENT DENSITY THEREON OF ABOUT 0.1 AMPERE TO 30 AMPERES PER SQUARE INCH TO THEREBY ELECTRODEPOSIT ALUMINUM ONTO THE SURFACE OF SAID ARTICLE AT A RATE NOT SUBSTANTIALLY GREATER THAN THE RATE AT WHICH ALUMINUM DIFFUSES THEREIN, SUBSEQUENTLY COOLING SAID ARTICLE AND REMOVING ANY RESIDUAL SALTS FROM SAID SURFACE. 